Change in religions is inevitable. It is neither good nor bad, but inevitable. What else would one expect? In fact, if a religion is still alive it is going to be changing.

The story of the Buddha is always told and retold because it is a living story. In the same way, the Buddha's dharma is always growing and expanding because it is a living dharma. This doesn't mean that things that come in to the dharma later are better or worse. This is very important because there is tremendous bias in Buddhism where Buddhists either think "We've got the later true teachings of the Buddha which were given when he really know what he wanted to say" or "We have the original teachings of the Buddha, unadulterated, and not mucked up by everything that came later." Both views are biases. Religions will change. This is inevitable and should not be seen as good or bad. We just need to look at what the changes have been and to try and find the causes and conditions that brought them into existence.

Sunyata (non-substantiality, void, emptiness) is the reason things change. According to Nagarjuna, since everything arises and continues to exist by virtue of its relationship with other phenomena (known as "dependent origination"), no phenomenon is absolutely fixed, nor has an independent essence of its own. It is "empty".
In such a case, there is nothing in life that cannot be changed. Nothing exists entirely of its own accord, and no "thing" (including religion) is absolute and immutable. The universe (i.e. reality) is full of new opportunities at every moment.